The present invention relates to a pulse motor frequently used in printers and the like.
In printers, for example, a pulse motor (stepping motor) having a good controllability is often employed for scanning a print head, or driving a platen roll.
FIG. 2 shows an exploded oblique view of a prior-art pulse motor using a permanent magnet for the rotor.
The rotor 1 of this pulse motor comprises a permanent magnet divided and magnetized along its circumference. A rotor shaft 2 is fixed to the center of the rotor 1 by means of a collar, not shown. Provided to surround the rotor 1 are a pair of drive coils 3 and 4, and inner yokes 5 and 6 are disposed between the drive coils 3 and 4. Provided to surround the drive coils 3 and 4 are outer yokes 7 and 8. A flange 9 and a mounting plate 11 are provided at the top and at the bottom. The flange 9 is provided with a bearing 10 for accepting and supporting the rotor shaft 2. The mounting plate 11 is provided with a similar bearing, although not illustrated as such.
The two yokes 5 and 6 comprise annular magnetic plates 5a and 6a, respectively, and are provided, at their inner peripheries, n/4 (n being the number of steps per rotation of the rotor) magnetic poles 5b and 6b, respectively, formed by bending so that they are parallel with the rotor shaft 2. In the drawings, the magnetic poles 5b of the inner yoke 5 are shown to be bent upward. The magnetic poles 6b of the inner yoke 6 are bent downward. The magnetic poles 5b of the inner yoke 5 and the magnetic poles 6b of the inner yoke 6 are arranged so that their phases are 90.degree. offset relative to each other.
Magnetic poles of the same shape are provided in the same number on the lower surface of the outer yoke 7 and the upper surface of the outer yoke 8. Magnetic poles, not shown, on the outer yoke 7 are provided to confront the magnetic poles 5b on the inner yoke 5 so that their phases are 180.degree. offset relative to each other. Similarly, the magnetic poles 23 on the outer yoke 8 are provided to confront the magnetic poles 6b on the upper yoke 6 so that their phases are 180.degree. offset relative to each other. The drive coils 3 and 4 are of such a configuration that the coil 14 or 16 are wound on the bobbins 13 and 15.
The outer yoke 7, the flange 9, and the outer yoke 8 and the mounting plate 11 are secured by spot welding or the like. The inner yokes 5 and 6 and the outer yokes 7 and 8 are stacked so as to surround the rotor 1, and the outer yokes 7 and 8 are fitted over the bobbins 13 and 15. The assembly is thus completed.
In the pulse motor of the above construction, when predetermined alternating currents with their phases offset relative to each other are supplied to the drive coils 3 and 4, the rotor 1 rotates at the corresponding period. The magnetic flux generated by the drive coil 3 passes through the magnetic poles 5b of the inner yoke 5, passes from the central portion of the inner yoke 5 to the outer portion, and passes through the outer periphery of the outer yoke 7, and then through the top surface toward the bearing 10, and then through the magnetic poles, not shown, on the lower surface of the outer yoke 7, and then across the air gap, and then enters the rotor 1. The magnetic flux then passes across an air gap and returns to the magnetic poles of the inner yoke 5. A similar magnetic path is formed for the drive coil 4, with the inner yoke 6 and the outer yoke 8 being included in the path.
The rotor 1 is driven in such a direction that the magnetic paths are shortened. By supplying the drive coils 3 and 4, with alternating currents with their phases 90.degree. relative to each other, a continuous drive force of 4 steps per period of the alternating currents are derived and the rotor 1 is thereby rotated.
In recent years, size reduction, cost reduction and improvement in performance are required in connection with printers and the like, and attendantly the pulse motors used in these equipment are required to be of smaller size, lower cost have a higher torque and higher rotational speed.
An attempt has been made to increase the torque without increasing the outer dimension of the motor itself by using rare earth magnets in place of conventional ferrite magnets for the rotor 1. But then the low-price feature of the permanent magnet type pulse motor using permanent magnets for the rotor would be lost.
Another attempt to increase the torque is to increase the current flowing through the drive coils up to their rating. But then the problem of heat generation in the pulse motor itself or the drive circuit occurs. A countermeasure is to provide a heat sink, cooling fans or the like for the pulse motor or the drive circuit to increase the cooling efficiency. This then raises the cost, and the size of the motor and the peripheral circuits. A further attempt made was to optimize the parameters, such as coil constants and the drive currents, to maximize the output with the limited outer dimension. But this also has a limitation.